Gerontological research has produced numerous examples of the age-related decline in the immune system. Nevertheless, despite evidence that infections are a major cause of morbidity and mortality in the elderly population, the age-related immune response to infectious agents per se has not been studied. The present proposal will compare the T cell response of old and young mice to influenza. This virus has been shown to elicit a murine response which parallels the human response in both specificity and kinetics. Preliminary experiments indicate that aging mice show a diminished cytotoxic T cell response to influenza. The aims of the present study are to analyse possible causes for this decrease and to demonstrate in vivo consequences of the diminished response.